Objectives: To assess the changes in measures of fatigue after meals of varying composition, and to compare the relation of the fatigue with changes in the plasma tryptophan:large neutral amino acids (Trp:LNAA) ratio.
Subjects: Sixteen healthy volunteers were recruited from staff and students at the College.
Design: Subjects were tested for central and peripheral fatigue using a visual analogue scale, flicker fusion frequency, grip strength, reaction time and wrist ergometry. In addition, plasma amino acid concentrations and Trp:LNAA ratio were determined. Measurements were made before, and at 1, 2, 3 and 4 h after drinking one of three liquid test meals. The meals were isoenergetic (1672 kJ) and were of mixed carbohydrate, fat and protein, or of pure carbohydrate or pure fat.
Setting: Department of Human Nutrition.
Results: Subjects consuming the pure carbohydrate meal reported more subjective feelings of fatigue and had slower reaction times. Aspects of central fatigue were greater in subjects consuming a pure fat meal. The Trp:LNAA ratio was depressed in those consuming a pure fat or mixed meal and raised only after pure carbohydrate.
Conclusions: Central and subjective fatigue may be influenced by raised plasma free tryptophan to competitor amino acid ratios induced by carbohydrate intake but other aspects of central arousal are affected by fat intake.